The present invention is directed towards compositions having a first zeolite layer, a second layer that may be permselective, catalytic, or both, and a substrate or support situated therebetween.
Molecular sieve membranes such as zeolite membranes have been proposed for uses such as molecular separations, reactions, and combined separations and reactions. Such membranes, sometimes referred to as molecular sieve membrane composites or zeolite membrane composites, comprise a zeolite layer, or other inorganic layer capable of molecular separations, on porous or solid substrates. The membranes may also include optional growth enhancing layers, seed layers, and reparation layers. Layers in contact with or penetrating into the support may be used to reduce support invasion by the zeolite layer. Additionally, the zeolite layer may have a catalytic functionality present in the layer itself, in some cases inherently, or in the form of a proximately-located catalytically active material.
However, some separation, reaction, or combined separation/reaction processes cannot be performed with zeolite membranes known in the art. Some examples of such processes include cases where the catalytic functionality is in the form of catalyst distributed, sometimes with a binder, on or in to one or more layers of the membrane. Unfortunately, such membranes would be subject to catalyst attrition. Binder contamination is also a problem, especially in cases where the binder is itself catalytic.
Zeolite compositions capable of combined separation and reaction are known in the art. One example of such a composition uses conventional catalyst particles on or near the separation layer so that specific products and reactants can be selectively added or removed from the reaction zone. In other examples of such compositions, the catalytic functionality, which may be in the form of catalyst particles, is located within the zeolite layer.
It is desirable to apply compositions combining reaction and separation functionalities to processes involving staged separations. In staged separations, one or more reactants are separated from a reactant source by a zeolite or zeolite-like layer. One or more additional layers may be used to further separate the reactants, reactants and products, or products formed in or between the layers. Compositions of two or more layers, one of which is a zeolite layer, in combination with catalytic functionality and capable of staged separation are not known in the art. Obstacles to forming such compositions include materials-based incompatibilities.
Therefore, there remains a need for zeolite membrane composite compositions having a first zeolite layer, a second layer that may be permselective, catalytic, or both, and a substrate situated therebetween, the composite being capable of separations, reactions, and combined reaction and separation. There is also a need for zeolite membrane composite compositions having a permselective zeolite layer in contact with a support, the support having a catalytic functionality contained thereon.
In one embodiment, the invention is a zeolite membrane composite composition comprising:
a substrate in contact with a mesoporous, crystalline or polycrystalline intermediate layer of a catalytic functionality ranging in size from about 0.1 to about 100 micrometers and nanocrystalline or colloidal-sized zeolite, the intermediate layer being in contact with a dense layer of columnar zeolite crystals having pore openings wherein 99% of the columnar zeolite crystals have at least one point between adjacent crystals that is less than or equal to 20 xc3x85.